Sound-translating system



y 5, 1930- H. c. HARRISON 1,757,459

SOUND TRANSLATING SYSTEM 7 Original Filed May 2, 1923 2 Sheds-Sheet 1 za F H\ g 9 2/ /6 gf I 20 y 6, 1930' H. c. HARRISON 1,757,459

SOUND TRANSLATING SYSTEM Original Filed May 2, 1925 2 Sheets-Sheet 2 Patented May 6, 1930 UNITED STATES PATENT orrlca "nnnn roanannxson, or ron'r WASHINGTON, *inigw YORK, assrenon. 'ro WESTERN 'f'ELECTRIU-COMZBQY mqoarona'rrm, OI NEWIOBK, 1w. 1., a oonrona'rron or Application Med May 2, 1923, Serial No. 63%!) This invention relates to sound translating systems such as tubular horns emplo ed, for example, with phonographs and lou speaking telephone receivers.

An object of this invention is to provide a high efliciency coupling between a vibrating member of small area and a column of air of much larger area.

Another object of this invention is to reduce reflection losses in a loud speaking horn.

The problem of coupling a vi rating member. such for example, as the diaphragm of a telephone receiver to a column of air of larger area. involves large reflection losses unless special precautions are taken to adapt the mechanical impedance of the vibrating member to that of the column of air desired. In employing a tubular horn for such a purpose the problem involves starting with a sound wave of high velocity and small area and ending with a sound wave of low velocity and large area. The rate of transformation of the wave in such a system depends upon the rate of change of the mechanical im edance of the channel, which depends upon t e area, the elasticity and the density. It has been found that a certain change in mechanical impedance per wave length is permissible with good operation and this invention therefore proposes to provide in a relatively small space a tapered mechanical impedance arrangement which will chan e by small steps from the impedance of tie vibratmg member to the impedance at the open end of the horn. A tapered change in impedance to develop from the small area of the vibrating member to the area equal to the mouth of the horn may be made in a comparatwely small distance if the average density of the translating medium is kept high, because with any maximum permissible change in mechanical impedance per wave length the rate of change per unit length is greater the greater the density of the medium in which the change in area takes place. In accordance with one form of this invention a rapld increase in the outline of the horn is made permissible by including within the horn a plurality of diaphragms of sufiicient thlckness and proximlty to maintain substantlally saunas-assass nate swam Renewed au ust 18, 1927.

constant. at a high value the average density of the translating medium until the desired increase in area is obtained. The horn is then extended over a small distance to support a plurality of diaphragms gradually decreasing in thickness .and increasing in spacing looking in the direction of the mouth of the horn, thereby producing gradual changes in the average density of the medium until the open air is reached. The diaphragm should be of a material of low elasticity such as thin rubber or soft aper. The tapered impedance may also e secured by non-vibrating plates within the horn and perforated to manner desired.

Referring to the drawings, Fig. 1 represents this invention employed for coupling a diaphragm of small areaand high density to a column of air of larger area and smaller density; Fig. 2 is a modification of Fig. 1 in .which the outline of both sections of the horn is curved; Fig. 3 is a modification of Fig. 1 in which the horn has a constant slope from end to end; Fig. 4 discloses an arrangement for providing an eflicient coupling between a diaphragm of lar er area and the small end of a large horn; ig. 5 discloses how the horn of Fig. 4 may be considerably shortened by the use of perforated plates of constant area; Fig. 5 represents a View with the plates in section looking into the mouth of the horn of Fig. 5; Fig. 6 discloses how the horn of Fig. 4 may be shortened by the use of perforated plates of varying area; Fig. 7 discloses how the horn of Fig. 4 may be shortened by the use of imperforate diaphragms; and Fig. 8 discloses this invention employed in the mouth piece of a telephone transmitter.

Referring more particularly to Fig. l a tubular horn 9 is disclosed having located at its small end the diaphragm 10 of a telephone receiver or a phonograph reproducer, indicated by the block 11. Such a diaphragm is usually of considerably smaller area than the mouth of the horn associated therewith than air several thousand times as a rule, made usuall of steel or other metal in the case of a .te ephone receiver and in the case that of air.

therein a arge number of diaphra of a honograph reproducer is made of materi such as wood or mica. The mechanical impedance of the diaphragm 10 is therefore quite diflerent from the mechanical impedance of a column of air ual to its area and large reflection l wi take lace when vibrating this column of air an ess special g recautions are taken to adapt the mechanical impedance of the vibration member to that of the column of air.

It has been found, that a certain change in mechanical impedance per unit-wave len is permissible with good operation and t is invention therefore proposes to PIOVldB a tapered mechanical impedance arrangement which will change b small steps from the impedance of the vi rating member to the impedance looking out into the air. This tapered impedance arrangement, to develop from the area of the vibrating member to the lar er area at the mouth of t e horn, may be ma ic in a comparatively small space if the average density of the translating medium is kept high because the maximum perm1ss1- ble chan e in mechanical impedance per wave length s greater the greater the density. The chan from the area at small end of the horn to te area at the open end of the horn is made ra idly in section 14 by employirig with constant spacing but gradually increasing in area. The thickness of the diaphragms 13 should be such as to maintain substantially constant the average density of the translating medium from the diaphragm 10 until the oint 15 is reached. The thickness of the aphragms 13 will depend not only upon the material of which they are made but also upon the average density of the diaphragm 10 and the layer of air on each side. By having the diaphragms 13 gradually increase in area from ad acent the diaphragm 10 until the maximum area of the horn is reached, the tapered chan e in mechanical impedance due to gradua increase in area may be made ra idly because the change in impedance is beln made in a medium of relatively high density compared to The change in impedance from the dia hragm 10 to the point 15 may therefore al ow the area of the horn to be rapidly increased in area without danger of appreciable reflection losses or a reduction in the quality of transmission of the sound waves.

The maximum permissible chan e in mechanical impedance per wave lengt depends upon the quality desired in any artlcular case. It has been found for exam Fe that the impedance may be changed by a actor 2500 per wave len without impairing the transmission.

After the point15 is reached the problem still remains to taper gradually the densi of the translating me ium from the hig density maintained in the portion 14 until the density has been tapered to the densitg of air. This is taken care of in section 1 of the horn by providing a relative-16 large number of diaphragms 1 of gradu y diminishing thickness from the. point 15 until the open end ofv the horn is reached and also radually increasing in spacing 1 00 in the direction of the mouth of the horn. arrangement therefore enables the density of the translatin medium to be rapidllyrtapered from the hig density of the 'ap agm 10 to the density of the air without im airing the high quality transmission desi In the above described horn with the diahragms 13 and 17 the translating medium behaves substantially as though it were a homogeneous medium of constant density as long as there'are several, preferably more than seven lumped masses r wave length.

The diaphragms 13 and 1 should preferably be ma e of material which does not have appreciable elasticity so that the dia hragms 13 and 17 will act as substantialiy pure masses for all frequencies within the speech range. For example the diaphragms 13 and 17 may be made of cloth im regnated with rubber or may be made of ru ber entirely or of soft paper. In any case these diaphragms should be preferably air-tight to prevent the circulation of airbetween the various compartments. The periphery of each diaphragm may be attached to the inner wall of the horn in any suitable manner.

The arrangement of the diaphragms with the air compartments between also acts as a low ass filter comprising series masses (the diap ragms) and shunt elasticities (the air between the diaphragms) so as to suppress objectionable high frequencies above the cutoff frequency, such as a art of the scratching noises and the like. 'Fhe spacing and the weight of the diaphragms 13 and 17 will therefore depend upon the cut-off frequency desired in any particular case.

Fig. 2 is a modification of Fig. 1 in which a born 20 is disclosed havin a section 21 rapidly increasing in area mm the diaphragm 22 until the point 23 is reached. This section 21 is filled with a large number of diaphragms 24 constructed in substantially the same manner as the section 14 of the horn shown in Fig. 1. The section of the horn from the point 23 to the open end instead of being straight as in Fig. 1 is shown sllghtly curved to allow a gradual increase in the area of'the horn even after the point 23 is reached. This section 25 is similar to the ness and increasing in spacing looking in the direction of the mouth of the horn to produce the desired taper change in density.

Fig. 3 illustrates a tubular horn 28 which has a constant rate of increase in area from its small end to the large end. In this figure, the soundresponsive diaphragm 29 is coupled to the column of air at the mouth of a horn by a plurality of diaphragms 30 of gradually increasing area and diminishing thickness looking in the direction of the large end of the horn. This arrangement is therefore a modification of that previously discussed in that the tapered change in density is made simultaneously with the tapered change in area. The maximum permissible chan in mechanical impedance per unit lengt along the horn 28 will depend as previously discussed upon the wave lengths of the sound waves to be transmitted thereby, since for a given wave length there is a maximum permissible change in mechanical impedance per unit length beyond which serious reflection losses will occur. The thickness of the diaphragms 30 will therefore depend upon the density of the sound responsive diaphragm 29 as well as upon the maximum frequency of the sound it is desired to transmit efliciently along the horn.

It has been previously disclosed in my prior application Serial No. 610,977 filed January 6, 1923, that a satisfactory arrangement for coupling a sound responsive diaphragm to the small end of a lon horn is by means of an air chamber of pre etermined size which acts as an auto-transformer for steppin down the mechanical impedance of the soun responsive diaphragm to the impedance at the small end of t e horn. It is disclosed for reference purposes in Fig. 4 in which the diaphragm 31 is coupled to the small end of a long horn 32 by an air chamber 33 designed so that the mechanical impedance of the dia hragm 31 as seen from the small end of the orn is substantially the same as the mechanical impedance of the column of air looking in the direction of the open end of the horn from the same point.

The length of the horn 32 may be considerably reduced and decreased to a length of 1.3 or less by employing the present invention for producing a tapered change from the density of the sound responsive diaphragm to the density of air.

In Figs. 5 and 5, for example, the sound responsive dia hragm 33 is located at the small end of t e liorn 34 and matches the impedance at this point because it has been made high by the use of a plurality of plates 35 of constant area but perforated, the number of perforations 48 per unit area increasing from plate to late looking in the direction away from t e sound responsive diaphragm 33. The efiective density of the wave transmitting medium depends of course upon the plate spacing and the numher and size of the perforations so that by increasing the number or size of the perforations in the plates looking in the direction of the mouth of the horn the average density of the translating medium for the sound vibrations may be rapidly decreased from the high density of the diaphragm 33 to that 0 air.

The size of the holes will depend upon the thickness of the lates and for thin plates of the thickness 0 paper they may vary from in. to A; in. or more while for thick plates they ma have a diameter two or three times the thic ness of the plates. In some cases only a few holes in each plate will be necessar while at other times a larger number suc as thirty or more will be needed. After the point 36 is reached the horn 34 is given a tapered outline to increase gradually the area from the oint '36 until the mouth of the horn is reac ed. Care must be taken of course to insure that the change in mechanical im )edance er unit length along the section 3 of the orn is not greater than the maximum permissible change in mechanical impedance for the desired wave lengths to be efliciently transmitted. The change in area between the point 36 and the open end of the horn may, of course, be made still more rapidly without impairing the efficiency of transmission by employing im erforate diaphragms as in Fig. 1 of adual y diminishing thickness and gra ually increased spacin looking in the direction of the mouth of the orn.

Fig. 6 is a modification of Fig. 5 in which the section 38 employed for changing from the high density of the diaphragm 39 to that of the density of air is made while simultaneously increasing the area. The perforated plates 40 are similar to the plates 35 of Fig. 5 except that looking in the direction of the mouth of the horn their area is increased in addition to the increase made in the number or size of the perforations. This gradual increase in the area of the perforated plates 40 may be made safely providing the maximum permissible change in impedance per wave length is not exceeded.

The form of this invention disclosed in Fig. 7 differs from the form shown in Fig. 5 only in that a plurality of imperforate diahragms 41 are disclosed in section 42 of the lOIl'l for tapering the densit of the sound responsive diaphragm 43 to t at of air so that t e imperforate diaphragms act simultaneously to the perforated lates 35 of Fig. 5. The diaphragms 41 are s IOWII to be of gradually decreased thickness and increased spacing looking in the direction of the mouth of the horn for the reasons hereinbefore given.

The forms of this invention discussed up to this point have dealt with the application of this invention to a coupling means between a sound actuated diaphragm and a column of mechanical im adaptedto the mechanical im ance at the I open end of the horn. These iaphragms 46 looking from the open end of the horn in the direction of the transmitter diaphragm-47 are gradually decreased in spacing and increased in thickness to taper the impedance at the open end of the horn to the impedance of the transmitter diaphragm 47 or to the dance of the associated carbon chamber without exceeding the maximum rmissible change in impedance per unit ength, so as to produce an eflicient translating system;

Perforated plates may be used instead of the diaphragms 47. Other modifications of this invention will be a parent to those skilled in the art without eparting anywise from the invention as defined in the appended claims.

What is claimed is:

1. A sound translating member comprising a horn having a plurality of diaphragms of low elasticity s aced along a portion of the length of said orn and in contact with the inner surfaces of said horn.

2. A mechanical network of tapered impedance comprisin a tubular channel havin a plurality of ine astic diaphragms space along a portion of its. length and in contact with its inner surface.

3. A mechanical network of tapered impedance comprising a horn, a sound responsive diaphragm located near one end of said horn and being of high density compared to air. and a plurality of plates spaced on one side of said diaphragm within said horn of such dimensions and of such a distribution to give a tapereddecrease throughout the length of the horn in the average densit of the translating medium per unit length a ong the horn looking away from said diaphragm.

4. A sound radiating member comprising a tubular channel and a plurality of plates graduated in mass and spacing being s aced along and in contact with a portion 0 said channel.

5. A sound radiating member comprising a tubular channel and a plurality of plates graduated in thickness and spaced along a portion of saidchannel.

6. A sound radiating member comprising a horn and a plurality of plates graduated s pfiaking telephone receivers and they in thickness and spaced along a portion of said horn at duated intervals.

7. A soun radiating member comprisin a tubular channel, a diaphragm at one end 0 said channel and a plurality of plates s aced inwardly along the axis of a portion 0 said channel and graduated in mass to taper the density per unit length alo the channel by small steps from the density of said diaphragm to that of air.

8. A sound radiating member comprising a horn, a lurality of plates spaced along a portion 0 said horn and having the same thickness but varyin in area and a plurality of plates spaced a on and in contact with another portion of said am with a tapered spacing. v

9. In combination, a horn, a sound responsive diaphragm located near one end of said horn, and means within the horn for tapering the impedance of said diaphra to the impedance of the air at the mout of said horn without causing losses within said horn.

10. In combination, a horn, a sound responsive diaphragm locatednear one end of said horn, and means within the horn for compensating for difference in mechanical impedance caused by any difierence in area be tween the area of said diaphra and the substantial reflection I ing substantial reflection losses.

11. In combination, a horn, a sound responsive diaphragm located near one end of said horn, and means within said horn for compensating for the difierence in mechanical impedance caused by said diaphragm having a reater density than that of air without causing substantial reflection losses within said horn.

12. A horn comprising a sound res nsive diaphragm at one end thereof, a' p urality of inelastic diaphragms adjacent said first diaphragm of constant thickness but progressively increasing in area roceeding away from the dia hragm, and a urality of other diaphragms ocated in a di erent part of said horn of progressively diminished thickness looking away from said first diaphragm.

13. A loud speaking horn having a sound responsive diaphragm located near one end, a plurality of diaphragms of constant thickness but progressively increasing in area located within said horn and adjacent said first diaphragm, and a plurality of other diaphragms located in another portion of said horn with progressively increased spacing and progressively diminished thickness looking away from said first diaphragm, both sets of said diaphragms being composed of a material having negligible elasticity compared to the elasticity of said first diaphragTn.

s0 distributed that there are at least seven per wave length.

-15. An acoustic device comprising a tubular member havin a sound transmitting medium therein, a plurality of diaphragms dividing said member into a. plurality of separate substantially air-tight chambers, said diaphragm being adapted to vibrate solely in response to forces transmitted through said medium, and means for impressing sound waves on said medium at one end of said member.

16. An acoustic device for uniformly transmitting sound waves covering a ran e of frequencies, comprising a tubular mem er, having a sound transmitting member therein, and acoustic impedances comprising lumped masses along the length of said member dividmg said member into a. lurality of separate su stantially air-tight c ambers, said masses being adapted to vibrate solely in res onse to forces transmitted through said me um.

In witness whereof, I hereunto subscribe my name this 30th day of April, A. D. 1923.

HENRY C. HARRISON. 

